Fix boot GHC for `ghcup compile ghc`

It turns out that passing `ghcup compile ghc -b ghc-9.6.5` actually silently falls back to `ghc` in PATH: https://gitlab.haskell.org/ghc/ghc/-/issues/24682

So we either have to:

- let the build fail and document that it needs to be an absolute path
- normalize it to an absolute path